The invention relates to digital communications systems generally and, more particularly, the invention relates to communications systems receiving packetized information streams.
Information distribution systems comprise one or more information servers that provide information to a plurality of subscribers via a distribution channel. For example, a cable television system includes head-end equipment to transmit audio-visual information to subscriber equipment via an analog or digital communications channel.
Subscriber equipment connected to an information distribution system typically injects some electromagnetic noise back into the distribution channel. For example, cable television subscriber equipment is susceptible to the electromagnetic noise generated by various electrical and mechanical devices used within a subscriber""s household, such as air conditioning units, fans, appliances and the like. The subscriber equipment may operate to sum the generated electromagnetic noises and inject the summed noises back into the information distribution system. As new subscribers are added to the information distribution system, the total amount of injected noise increases proportionately, a phenomena known as xe2x80x9cnoise funneling.xe2x80x9d Thus, the amount of noise in the information distribution system is proportional to the number of subscribers and the amount of noise generated by the subscribers.
In the case of an interactive information distribution system, such as an interactive cable television system, the head-end may provide information to subscribers via a forward channel and receive information from subscribers via a back channel. In such an interactive system, the head-end equipment will also comprise a back channel modem receiver that is used to receive information (such as program requests) from subscriber equipment. It is known to implement a forward channel and a back channel on the same physical medium by using mutually exclusive spectral regions for each channel.
The modem receiver must be able to retrieve and process valid information from the back channel in a timely fashion. Moreover, the information received via the back channel is typically characterized as a burst mode data transmission. Thus, the modem receiver must be able to rapidly process the retrieved information to produce a desired result. Unfortunately, since the level of noise within the system is not constant, the signal threshold of the modem receiver must be adjusted to adapt to the noise level.
The invention is a method and apparatus for adapting a signal detection threshold level in response to a varying background noise level in an information signal. Signal excursions beyond the logic threshold are indicative of the presence of an information packet in the information signal.
Specifically, a digital data receiver according to the invention comprises a peak detector, coupled to receive a digital data input signal; for producing therefrom a bandwidth limited signal associated with a noise floor; an offset adjuster, coupled to the peak detector, for imparting an offset to the bandwidth limited signal associated with a noise floor to produce a threshold signal; and a comparator, coupled to receive the digital data input signal at a first input and the threshold signal at a second input, the comparator producing an output signal indicative of the digital data input signal having an amplitude level exceeding the threshold signal amplitude level, the signal excursion beyond the threshold level indicating a detection of an information packet.
In one embodiment of the invention, a digital data receiver includes an information packet processor having a relatively low dynamic range, e.g., a dynamic range tending to be insufficient for high reliability information packet detection and reception. In this embodiment, the digital data receiver includes an information packet detector having a relatively high dynamic range that is operatively coupled to the information packet processor such that the combined information packet detector and information packet processor provides substantially high reliability information packet detection and reception.
Specifically, a digital data receiver according to the invention comprises a packet detection circuit, coupled to receive a digital data input signal, for detecting the presence of an information packet in the digital data input signal and providing an output signal indicative of the presence; and a packet processing circuit exhibiting non-optimal packet detection capabilities, coupled to receive the digital data input signal and the packet detection circuit output signal; wherein the packet processing circuit, in response to the packet detection circuit output signal indicating that information packet is present in the digital data input signal, processing the digital data input signal to retrieve the indicated packet.
A method of detecting an information burst in a digital information signal according to the invention comprises the steps of: establishing a noise floor associated with a noise component of the digital information signal; modifying, in response to the noise floor, a logic threshold level associated with the information burst; evaluating, using the modified threshold level, the digital information signal; and providing indicia of digital information signal excursions beyond the threshold level.